When light is projected from an underwater object to an underwater imaging device (i.e. camera or human eye), the light is either attenuated (in the form of absorption and scatter), or transmitted directly to the imaging device without perturbation. Since an image is formed only by light traveling in a straight line from object to imager, the light that is transmitted directly is called image-forming light. Light that is absorbed or backscattered never reaches the imager, and light that is forward-scattered into the imager tends only to blur or saturate the image rather than enhance it. Since the purpose of a transmissometer is to measure only the image forming light over a specified path between a light source and receiver, it is the sensor of choice for imaging system performance assessment.
The conceptual design of a transmissometer is basic. An ideal transmissometer 100 is shown in FIG. 1 according to prior art. Ideal transmissometer 100 creates a collimated beam 101 and measures the amount of collimated light (i.e. image forming light) that is received over a finite path length. A point source 103 is projected onto an objective lens 105 to form collimated beam 101. Beam 101 is projected over a finite path length of water. A second objective lens 107 is used to image collimated beam 101 to a point 109 on a light detector (if a laser is used as the light source, there is no need for the lenses). Transmissometer 100 is ideal because it creates a perfectly collimated beam with an ideal point source, and measures only the unperturbed image forming light with an ideal point receiver that has a zero acceptance angle.
Since real transmissometers cannot have ideal point sources and ideal point receivers, some of the forward-scattered light from particles in the measurement path will reach the detector and “cloud” the measurement. The problem with real transmissometers is twofold. First, an ideal point source cannot be achieved, thereby producing some divergence angle to the collimated beam. Second, an ideal point receiver cannot be achieved, thereby producing some finite acceptance angle for light entering the receiver.
In order to overcome these limitations, the optical design of a real transmissometer should be assessed carefully to optimize its performance and limit its cost. Typically, one of two optical design approaches can be taken for a real transmissometer. The first is the “cylindrically limited beam” approach (CLB), and the second is the “diverging collimated beam” approach (DCB).
A CLB transmissometer 200 is shown in FIG. 2 according to prior art. It consists of a projector, a measurement cylinder (or path), and a receiver. A light source 217 (i.e. LED) is placed behind a field stop 205 in a projector 201. An objective lens 203 is used to image field stop 205 at the receiver entrance aperture, where a second objective lens 207 is placed. A receiver field stop 209 is sized and placed in front of a light detector 211 (i.e. photodiode) such that the apparent position of the projector aperture in water is imaged within the boundaries of receiver field stop 209. The defining feature of this type of instrument is that a lossless cylindrical beam is created between objective lens 203 of projector 201 and objective lens 207 of a receiver 213. Projector and receiver apertures are the same size, allowing for very long path lengths to be implemented, if necessary. The greatest angle a ray can be deviated and still be accepted by this type of system is indicated by an angle θS 215 in FIG. 2. Angle 215 is specified by the ratio of the beam diameter to one half the beam length. It is this ratio of beam diameter (or beam radius) to beam length that determines the percentage error of the instrument (i.e. the greater the radius to length ratio, the more forward scattered light that is accepted by receiver 215).
FIG. 3 shows a diverging collimated beam (DCB) transmissometer 300 according to prior art. It consists of a projector 301, a measurement cylinder 303 (or path), and a receiver 305. Projector 301 and receiver 305 elements are the similar as CLB transmissometer 200, but with transmissometer 300, projector and receiver field stops 307 and 309 are placed on the focal planes of projector objective and receiver objective lenses 311 and 313, respectively. This instrument type truly attempts to mimic the ideal transmissometer by providing a point source at projector 301 and a point detector at receiver 305. Since there is divergence out of projector 301, the receiver aperture should be larger than the projector's aperture. Performance of this type of transmissometer is not based on the radius to length ratio, but rather on how well it can mimic the ideal transmissometer with a point source and receiver.
Beam transmissometers are typically expensive scientific-grade instruments. However, a beam transmissometer is often used in applications in which the beam transmissometer is not easily retrievable after the completion of the application. (One example is the determination of a beam attenuation coefficient of seawater by launching a probe from a submarine.) Thus, there is a real need for providing a high quality beam transmissometer that may be inexpensively manufactured in order to be expendable if needed.